INTRODUCTION

Soft rot (Erwinia carotovora ssp. atroseptica) and dry rot (Fusarium sulphureum) are serious storage diseases which must be considered in potato breeding. Resistance of potato cultivars and breeders' selections to both pathogens has been investigated. So far, resistant cultivars have not been developed. Differences within Solanum tuberosum are slight and are restricted primarily to grades of susceptibility.

MATERIALS AND METHODS

Soft rot: Tubers of 269 cultivars and breeders' selection were evaluated during 1983. These were divided into five groups according to origin. There were 42 cultivars, 116 S. tuberosum (tbr) breeders' selections, 66 hybrids of tbr x S. tuberosum ssp. andigena (adg), 12 hybrids of cv. Butte x S.microdontum (mcd) and 33 hybrids of tbr haploids x S. phureja (phu) or S. stenotomum (stn). A second test in 1983 and two tests in 1984 were conducted with a limited number of clones, including some which were very resistant.

Erwinia carotovora ssn. atroseptica (Eca) was isolated on Stewart's pectate medium from potato stems with typical blackleg symptoms. Dilution of Eca was made with sterile distilled water to obtain 5 × 106 colony forming units/ml. Ten ml of inoculum were injected at a depth of 2cm into each wound. The amount of tuber rot was calculated as the volume of decayed tissue (cm3) using the formula of Sorensen & Sparks (1980) for determination of the volume of bruises.